In oil industry, metal pipelines are the main means of transportation and distribution of hydrocarbons. This is because they represent the most economical and safe solution for transferring over long distances and continuously outperforming transport efficiency by car-trunks (pipes), tank-trucks, trains and tankers. The need to achieve a timely supply of hydrocarbons has contributed, over time, to the installation throughout oil producer and consumer countries, of large networks of buried pipelines that during their exploitation process modify their technical and operational characteristics by several factors such as: deterioration caused by the environment surrounding them; pressure and temperature of the transported products; metal aging and its insulation, micro-tectonics movements, as well as the state of the cathodic protection system.
However, there are human factors that have impact in pipeline degradation such as: the improper handling during installation; inefficient state pipeline inspections and lack or poor maintenance works [Parker and Peattie, 1999]. All the aforementioned factors have as consequence the destruction of the insulation coating, development of corrosion and abrasion processes, reduction of wall-thickness of the pipeline and fractures. The effects of the deterioration of the pipes are reflected in economic loss and environmental damage.
Moreover, to ensure the integrity and operation of pipelines, methods of internal and external inspection are used. Internal methods are mainly used to evaluate fracture and corrosion on the walls of the pipelines. While to estimate damages in the coating and the cathodic protection system condition, nondestructive external electrical and electromagnetic methods are applied. The coating inspection of pipelines allows to detect areas or points of metal exposed to direct contact with the surrounding subsoil, being these areas or points of contact areas where corrosion is generated; besides, the inspections permit preventing metal corrosion avoiding breaks in pipelines and ecological disasters by hydrocarbons spills, thus reducing economic losses for damages, as well as higher costs in corrective maintenance.
Due to the pipeline being buried a few feet from the ground surface, the inspection of its coating can be done indirectly using electrical and electromagnetic methods.
Electrical methods such as CIPS (Close Interval Pipe-to-Soil Potential) [Pawson, 1998] and DCVG (Direct Current Voltage Gradient) [Masilela and Pereira, 1998] use electric field measurements of direct current to evaluate the effectiveness of the cathodic protection system and to locate defects in the coating. The pipeline-soil measurements are performed by synchronizing the on and off of the cathodic protection system and therefore, special equipment is required [Kho et al, 2007].
The disadvantage of these methods is their reliance on the resistivity of the medium as well as the pipeline depth, besides requiring prior knowledge of its position. However, these methods only provide qualitative information on damages of the coating and cannot be applied for the inspection of pipeline groups near or interconnected in shared rights of way.
To determine the path of the pipeline and the electrical quality of the coating, the PCM technique (Pipeline Current Mapper), which is based on measuring the magnetic field at the ground surface on the pipeline, is applied. The limitations of this methodology is that measurements can be performed at a maximum depth of 3 meters and cannot be applied in areas where there is pipeline congestion or crossing (separation between pipelines less than 4 times its depth) as overlapping of electrical fields occurs. Therefore, the determination of coating damage is qualitative.
The Superficial Electromagnetic Inspection Technology (TIEMS, in Spanish) allows one to assess the condition of the lining or coating of the pipelines from the emission of electromagnetic fields flowing radially on the environment that surrounds the pipeline and that are measured on the surface using sensors and synchronized receivers to the same frequency [Mousatov et al, 2004]. In the TIEMS it is approximated to the pipeline as electrical conductors of great length, presenting characteristics of capacitive and inductive reactors similarly to a transmission line when applying a periodic electrical signal. The processing of the measurements made on the surface allows to quantitatively obtain the value of the electrical resistance of the coating of the pipeline, identifying and delimiting the damaged areas. Likewise, to produce the electromagnetic field around the pipeline, it is used an external generator connected directly to the metal pipeline, proper management of their operational parameters are used: control type, magnitude, frequency and waveform of the transmitted signals, allow to extend the TIEMS scope as to be able to assess pipeline depths greater than 17 meters, interconnected pipelines and close share rights of via and to increase the resolution and inspection distances.
There are various types of commercial generators whose characteristics can be used to induce electromagnetic field around the pipeline, so it is important to note that the U.S. Pat. No. 6,051,977 patent, [Masuda et al, 2000] proposes a waveform generator capable of producing a variable signal which is connected directly to an amplifier block.
Moreover, in the generator proposed by Masuda, there is no way to perform a current control because there is not a block to monitor the system operating parameters. In the patent of [Masuda et al, 2000] a qualitative method is also proposed to evaluate through a moving vehicle, the coating condition of the pipeline, the proposed method is based exclusively on the detection of the magnetic field gradient along the pipeline axis and perpendicularly to it. There is no way to compensate variations in the depth of the pipeline, nor the effect of the surrounding environment, besides being applied only to detect damage in a single product.
However, in the design proposed by Pearson, in order to stabilize and keep constant the value of the output current [Pearson, 2004] it is considered a signal generator which controls the output power from a delta-sigma modulator, in this case there is feedback from the generator output to the modulator to achieve current stabilization. This design is characterized because after the power stage block exists a filtering block of the output signal of high power. The commercial equipment that protects the patent provides a combination of two or three simultaneous frequencies in sum sine (frequency combinations 4 Hz, 8 Hz, 98 Hz and 512 Hz), where the frequency used for the evaluation of the pipe is of 4 Hz, with preset intensities up to a maximum of 3 A and 50 Volts, so that it is possible to obtain a maximum output power of 150 watts. This maximum value limits the inspection of pipelines over 5 feet deep, and limits its scope along the pipeline.
Another option of generator is constituted by the ERA-MAX equipment, which provides one of six June preset frequencies to a maximum current of 200 mA with power output of 40 VA, the waveform of the output signal is rectangular and due to the low handling power, the scopes both in depth and length of the tube are smaller.